Topography is the shape of a surface, including its relief. The topography of the sea surface is influenced by both gravity and ocean circulation.
The actual shape of our planet is not a perfect sphere—it's not even a perfect ellipsoid (an effect of its rotation). If you measure the strength of the gravity field across Earth's surface, you will find local variations caused by irregularities in the crust, which produce an irregular surface called the geoid—the actual shape of Earth. To find the surface caused by ocean circulation, we mathematically remove the sea height caused by gravity (the geoid) and create maps of ocean topography. We then calculate the speed and direction of ocean currents similar to how meteorologists use atmospheric pressure maps to calculate winds.
Altimetry is the measurement of the height of the sea surface above (or below) some reference level. Every 10 days, the Jason-3 satellite measures heights of more than 90% of the world's ice-free oceans with the radar altimeter and complete 127 revolutions, or orbits, around Earth.
To measure sea surface height we must know the satellite's position in its orbit and the height between the satellite and the ocean's surface. We may also have to correct for variable amounts of water vapor in the lower atmosphere and free electrons in the upper atmosphere. Both of these can delay microwave pulses. Instruments on board the spacecraft allow us to do this.